An identifier generating method according to the present invention has a canonicalization process step of subjecting document data to a canonicalization process to correct fluctuation of expression; and an identifier generating step of, based on all or part of document data having been subjected to the canonicalization process in the canonicalization process step, generating an identifier uniquely specifying the document data or part thereof. When the fluctuation of expression is corrected by the canonicalization process, prior to the generation of the identifier in this way, the XML documents or RDF documents with the same meaning are transformed into documents in the same expression, and the identical identifier can be generated for them by a function of generating an identifier from a sequence of characters, as typified by a one-way function.